The supported list grows because people ask. If the board you ran is not on it, tell me about it and I will look into it.
▓▒░ The hard requirement
ATA boots a small FreeDOS machine in the caller's browser and hands the board a serial line, the way a modem would. That machine runs 16-bit DOS software, and only that. A 32-bit board, a Windows board, an OS/2 board, or anything written for a modern operating system is outside what ATA is built to do, and no amount of work on my end changes it.
Inside that boundary, almost anything runs. If it ran on a DOS machine and answered a modem, through a FOSSIL or straight off the serial port, it is a candidate. That covers nearly the whole field: the QuickBBS family, the WWIV and Telegard lines, the Citadels, the Fido-era boards, the big commercial packages, and the scene boards nobody has thought about in thirty years.
Note that plenty of these packages had later 32-bit or Windows successors. ATA wants the DOS one.
▓▒░ The second question
Getting a board to run is my work. Being allowed to give it to you is somebody else's decision, and it is the part that decides whether a package becomes a one-click install or stays something you bring yourself. To bundle it, one of these has to be true:
▓▒░ Where I draw the line
▓▒░ Making it quick
The more you can tell me up front, the faster it goes. Every one of these is something I would otherwise have to go and find:
A one-line request is welcome too. It will just take longer, because everything above is what I have to establish before a package can be packaged.
▓▒░ Write to me
Put the board's name in the subject line and I will know what it is about.
I cannot promise every board will make it. Some will turn out to be 32-bit, some will have an owner who says no, and some will have nobody left to ask and nothing that qualifies as abandonware. But every one gets looked at, and the ones that clear both bars get built.